You have a chance of receiving a piece of the Pirate set after defeating the final boss of Isle of Lost Skulls. The item you receive depends on your quest rating at the end of the quest: with C rank, you get cuffs or a belt; with B rank, you get a belt or a shirt; with A rank, you get a shirt or a hat; and with S rank, you get an item of the black set. A/B/C rank items may be duplicates; S rank items are always new (you get an A rank item if you have all of them). All items except S rank items can be bought with gold in the merch booth.